RESUMEN
Background: Daily pre-exposure prophylaxis (PrEP) regimen of tenofovir/emtricitabine (TDF/FTC) tablet is the standard of care for HIV prevention in Thailand. Event-driven PrEP (ED-PrEP), taking two loading pills 2-24 h prior to sex and two doses afterward, is considered an alternative regimen. This pilot study aimed to describe accuracy of and retention in ED-PrEP use among Thai young men who have sex with men (YMSM).Methods: A cohort of YMSM aged 15-19 years at risk of HIV infection, defined as reporting irregular condom use or having an HIV-positive sexual partner, was enrolled. All participants were provided oral TDF/FTC, condoms, and the "Raincoat" mobile application, which reminds users when to administer ED-PrEP. Participants attended clinic visits at Months 1, 3, and 6 and followed up by phone at Months 2, 4, and 5. Proper use of ED-PrEP was reported in person-months.Results: From August 2020 to July 2021, 36 YMSM with a median age of 18.7 (IQR, 17.5-19.4) years were enrolled. Thirty-two participants (88.9%, 95% CI: 73.9-96.9%) were retained at 6 months. No HIV incidence occurred during this study period. Of 197 person-months of follow-up, 69 (35%) involved sexual activity. In this group, 61% used ED-PrEP correctly. "Unplanned sexual activity" was the most commonly cited reason for incorrect use.Conclusions: YMSM who have infrequent sex had very high retention in ED-PrEP. However, about one-third of the sexually active period, ED-PrEP was not used correctly due to not taking the loading dose, suggesting that this regimen may not be suitable for unanticipated sexual activity.